1. Field of the Invention
The present invention relates to an electric vehicle which is propelled by an electric motor (hereinafter referred to as “motor”) energized by electric power when the rotational torque generated by the motor is transmitted to wheels through gears, and more particularly to a method of and an apparatus for controlling an oil temperature increase for an electric vehicle which employs an oil commonly used as a lubricating oil for the gears and a coolant for the motor, and an electric vehicle incorporating such a method and an apparatus.
2. Description of the Related Art
Heretofore, there have been commercially available electric vehicles which are propelled by a motor energized by electric power when the rotational torque generated by the motor is transmitted to wheels through a gear train.
The motor on such electric vehicles comprises, for example, a permanent-magnet synchronous motor including a rotor made up of a plurality of permanent magnets disposed circumferentially around a rotor core (iron core) and a stator having a plurality of coils disposed around respective stator core (iron core) teeth radially spaced from the permanent magnets.
The permanent-magnet synchronous motor has a magnetic pole sensor for detecting the positions of the permanent magnets of the rotor. The coils are supplied with three-phase alternating currents at advanced angles to produce a rotating magnetic field from the stator, thereby rotating the rotor.
Since the rotor is connected to a rotational shaft, the rotational torque of the rotor is transmitted through the rotational shaft to a gear train, i.e., a power train, and then to the wheels of the electric vehicle.
The gear train includes some gears that are housed, together with the motor, in a casing which contains an oil therein. The oil usually stays in vertically lower portion of the casing. When the gears in the casing rotate, the oil is driven upwardly to regions in the casing which are to be supplied with the oil. For details, reference should be made to Japanese Patent No. 3918631 and Japanese Laid-Open Patent Publication No. 2009-121549 (U.S. Patent Application Publication No. 2009/0127954 A1).
The regions in the casing which are to be supplied with the oil include meshing portions of the gears, sliding portions of the shaft, etc. When the oil is supplied to those regions in the casing, the meshing portions of the gears, the sliding portions of the shaft, etc. are lubricated and cooled by the oil.
The oil changes its viscosity in response to the temperature changes. When the viscosity of the oil increases as the temperature thereof decreases, the torque (driving torque) applied by the gears to drive the oil upwardly increases, resulting in an increase in the resistance to the rotation of the motor that is operatively coupled to the gears. The increase in the resistance to the rotation of the motor tends to reduce the efficiency of the motor, i.e., the efficiency with which to convert electric power into motor power and the efficiency with which to convert electric power into drive power for the wheels.
According to Japanese Patent No. 3918631, if the temperature of the oil is lower than a given temperature when the gearshift is placed in a parking position, then the coils of the stator are supplied with an electric current for producing no torque (0 N·m). The coils are now heated by the Joule heat to warm the oil for thereby lowering the viscosity of the oil.
For quickly starting the motor, it is preferable to increase the temperature of the oil quickly to a certain temperature.
According to Japanese Patent No. 3918631, the coils of the stator are supplied with an electric current for producing no torque (0 N·m) so that the coils are heated by the Joule heat due to their copper loss thereby to warm the oil, as described above. However, since the upper temperature limit of the coils is governed by the upper temperature limit (about 200° C.) of the wire insulations of the coils, there is a certain limitation to the upper rate at which the temperature of the coils rises, i.e., a certain limitation to the shorter time required to increase the temperature of the coils. In other words, it is difficult to warm cold oil quickly to a certain desired temperature. Particularly, when the motor is to be started at low temperatures such as an ambient air temperature at or below the freezing point, it will take a considerable time to warm the oil up to a desired temperature. Consequently, the oil temperature control technique according to the related art still remains to be improved.